U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position of a camshaft relative to a crankshaft. U.S. Pat. No. 5,107,804 further describes a VCT system within the field of the invention in which the system hydraulics includes a vane having lobes within an enclosed housing, the vane being oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft.
The control system for the VCT system of U.S. Pat. No. 5,002,023 utilizes a spool type control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. A VCT control valve, such as that of the aforesaid U.S. Pat. No. 5,002,023, has three functions: control the direction the VCT actuates; control the rate at which the VCT actuates; and stop the VCT at a specified phase position.
Stopping the VCT phase shifting elements in a specified position is accomplished by blocking the flow of hydraulic fluid into or out of the hydraulic chambers. The VCT phase shift direction is determined by selectively opening the appropriate exhaust passage allowing hydraulic fluid to exhaust one chamber and fill the other.
The VCT actuation rate is determined by governing the rate of flow from the selected exhaust passage. This is accomplished by the control valve, typically a spool valve, varying the flow area exposed at the exhaust port selected. The area exposed at the exhaust port is a function of two variables: the percentage of the hole in the sleeve that is exposed as the spool valve moves axially, and the radial gap between the spool valve and the sleeve. In a typical spool valve, the radial gap is increased with the spool valve stroke by a taper which is machined on the outside diameter of the valve. These spool and sleeve characteristics result in the flow area varying hyperbolically as a function of the spool valve stroke.
While the tapered spool valve design produces desirable flow characteristics, it also presents operational problems. One problem is that the tapered portion of the valve can potentially collect contamination. If the contamination wedges between the spool valve and sleeve, the valve may seize causing the VCT to lose control.